Process for the production of opaque protective coatings on aluminum and its alloys



Patented" oct. 21, 1941 PROCESS FOR THE PRODUCTION OF OPAQUE PROTECTIVE COATINGS ON .ALUMINUM AND ITS ALLOYS Max Schenk, Basel, Switzerland, assignor to the firm Ematal Electrochemical Corporation, New York, N. Y., a corporation of New York No Drawing. Application J my 10, 1937, Serial No.

11 Claims.

The present invention relates to a process for the production of opaque enamel-like, hard and protective coatings on aluminum and its alloys.

In the British patent specification No. 455,412 a process for the production of opaque protective and its alloys in a bath which contains a titanium compound.

Now it has been found that the titanium compound in the'bath can be entirely or partly replaced by a salt of zirconium and that by so doing opaque protective coatings of the same appearance and the same physical constitution, as also of the same chemical action with regard to circumstances causing corrosion can be obtained.

The process forming the subject matter of the present invention for the production of opaque protective layers on aluminum and its alloys is characterized by the feature that aluminum and its alloys are treated electrolytically in an aqueous bath containing a salt of zirconium, the said bath being free of strong mineral acids and their salts. The electrolytic treatment 'may be carried out either with alternating current or anod ically with direct current.

The term "salt of zirconium as used herein and in the appended claims, is to be understood as comprising the salts of the zirconium metal in which zirconium is present in the cation in contradistinction to and to the exclusion of salts of which zirconium is presthe zirconium metal in. ent in the anion.

For use as anions both inorganic and organic substances are suitable with the exception of those derived from the strong mineral acids. Amongst the inorganic acids the following are as, for examplag ycerine, glycol, polyvinylic alcohol, mannite, glucose, cane sugar, dimethylcellulose, dextrine and others.

Some of the ,additions such as, for example, acids become consumed in the course of time by In Switzerland July 13, 1936 j coatings on aluminum and its alloys is described. 1 p The said process consists in treating aluminum I suitable: Phosphoric acid, arsenic acid, boric acid,

case be replaced in the necessary quantities during or before the working operation.

The temperature of the bath during the treatment may be about 10-95 C. according to the degree of acidity of the said bath, the result which it is desired to obtain, and the amount of the additional substances and the voltage employed at any given time. The said temperature can be rendered uniform and kept constant in the bath by means of suitable mixing andcooling devices.

The articles to be treated are preferably, be-' fore the treatment, free from oil or grease, for example by means of petroleum, benzene or spirit and are thereafter subjected to a thorough etching operation. The etching substance may be alkaline (for example soda lye, soda, or basic phosphates) or acid (for example nitric acid or hydrofluoric acid).

The white protective coating possessing good covering power which consists principally of aluminum oxyhydrate obtained according to the invention, contains the oxides or the acid hydrate of zirconium oxyhydrate 'in very stable combination: The protecting coating provides, as is the case in the process according to the above named British specification, the same as andsometimes considerably 'higher resistance to corrosion thana pure aluminum oxide film and afiords quite exceptional protection to the metal forming the base.

It has further been found that aluminum and its alloys the surface of which has been coated according to any process whatever with a firmly adherent aluminum oxide film can be further treated according to the process forming the subject matter of thepresent invention.

The film can be coloured by means of organic dyestuffs ina dye bath containing aqueous or organic solvents, or by applying to it or coating it with coloured oils, metallic soaps, or varnishes the metal which dissolves and they may in this in any desired manner., For the production of a pure white appearance an after treatment may becarried out which consists in applying to the treated articles or'surface's over a certain time solutions of titanium, zirconium or thorium salts and hydrolising the salts retained in the coating by heating or by means of chemical agents. Just according to the preliminary treatment of the, surface before electrolising' according to the present process (producing a polish, sandblasted, etched, or fluted surface) bright enamel-likeor half-matt to fully matt eflects can be obtained.

By after-treatment with aliphatic acids or salts thereof with alkalis or strong bases, or withrsulphonate'd oils, the opaque protecting coating can be made water-repellent. Or again, the coating may be subsequently coated with suitable varnishes or burning-in resins.

- The articles or surfaces coming from the electrolising bath are preferably first washedv with water or dilute ammonia or dilute soda solution. or other alkaline liquors, then dyed, etc., and finally dried.

The following is an example of a suitable method of carrying the invention into effect.

A solution of 16 kilogrammes of commercial zirconium-sodium oxalate (Zr (C204) 2.3NQ2C204.H2C204.5H20) 16 kilogrammes of commercial boric acid, 16

kilograinmes of crystallised borax and 4 kilo-.

grammes of dextrine in 800 litres of filtered condensation water are employed as the bath. Electrolysis is carried out with direct current of 120 volts at a temperature of 72 C. at current densities of 6 amps. per square decimetre surface until the current density at full voltage has dropped to half the normal value.

I claim:

1. In the production of opaque,'hard and protective coatings on articles of aluminum and aluminum alloys, the step of subjecting an article, to be treated, as an electrode to the action of an electric current in an aqueous bath consisting of a solution of zirconium-sodium oxalate, boric acid, crystallised borax and dextrine.

2, A process of producing opaque, enamel-like, 'hard and protective coatings containing an oxide of zirconium on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath containing zirconium cations and containing anions which an oxygen containing andwill promote the formation of hard, adherent, opaque and enamel-like films on said airticles and-comprising anions "of the aliphatic carboxylic acids, and g -.electric current through said articles as the anode.

. 4. A process'of' producing opaque, enamel-like, hard and protective coatings containing an oxide of zirconium on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid-bath containing zirconium cations and containing anion's which are oxygen con taining and will promote the formation of, hard, adherent, opaque and enamel-like films on said articles and comprisi'ng those selected from the group consisting of the anions of phosphoric,

chromic, arsenic and boric acids, and passing:

electric current through anode.

- 5. A process of producing opaque, enamel-like,

said articles as the hard and protective coatings containing an oxide of zirconium on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath containing alkali zirconium cations and containing anions which are oxygen containing and will promote the formation of hard, adherent, opaque and enamel-like films on said articles, and passing electric current through said articles as the anode.

6. A process of producing opaque, enamel-like, hard and protective coatings containing an oxide of zirconium on articles of aluminum or its alloys, which comprises immersing said articles in an aqueous acid bath containing sodium-zirconium ticles and containing a mixture of oxalate and borate anions, and passing electric current through said articles as the anode.

8. A process according to claim 2, in which the bath temperature is maintained between 10 and 95 C. during' passage of the electric current.

9. A process of producing opaque, enamel-like, hard and protective coatings containing an oxide of zirconium on articles of aluminum or its. alloys, which comprises first forming on said articles a.

, film of aluminum oxide, then' immersing said articles in an aqueous acid bath containing zirconium cations and containing anions which are oxygen containing and will promote the formation of hard, adherent, 'opaque' and enamel-like films on said articles, andpassing an electric current through said articles as the anode.

10. An article of aluminum or its'alloys having an opaque enamel-like coating containing aluminum oxide and an oxide of zirconium, said coating being integral with the base and having been produced by the process of claim 2.

11. A process of producing opaque, enamel-like,

I hardand protective coatings containing an oxide alate, boric acid, borax and dextrine maintaining said bath at approximately 72 0., and passing a direct current having a current density of 6 amperes per square decimetre through said articles as the anode until the current density at full voltage has droppedto half the normal value.

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